Spring Flooding Underway, Expected to Worsen Through April

Source: NOAA

Thu, 2011-03-17 12:53

With spring flooding already underway over portions of the U.S., NOAA forecasters are warning the worst is yet to come. Almost half the country – from the north-central U.S. through the Midwest and the Northeast – has an above-average risk of flooding over the next few weeks, according to the annual spring outlook released today by NOAA’s National Weather Service. This week is also national Flood Safety Awareness Week, and NOAA has partnered with FEMA to encourage residents to prepare for this imminent threat.

The highest spring flood risk areas include the Red River of the North, which forms the state line between eastern North Dakota and northwest Minnesota; the Milk River in eastern Montana; the James and Big Sioux Rivers in South Dakota; the Minnesota River; the upper Mississippi River basin from Minneapolis southward to St. Louis; and a portion of lower New York, eastern Pennsylvania and northern New Jersey.

“For the third consecutive year, the stage is set for potential widespread, record flooding in the north-central U.S.,” says Jack Hayes, director of NOAA’s National Weather Service. “We’ve been coordinating with federal and state partners and high risk communities since December to raise awareness and help them prepare. All the ingredients are in place for major flooding so this situation should be taken very seriously. We’re asking citizens to stay informed and be prepared.”

Warm temperatures in the forecast this week could cause much of the snowpack to melt across South Dakota and southern Minnesota, setting off moderate to major flooding in eastern South Dakota next week. Minor flooding could begin this week on the Mississippi River and its tributaries over southeastern Minnesota and southwestern Wisconsin, leading to moderate to major flooding by early April.

In addition, a series of storm systems are forecast to move across the region during the next two weeks, which could bring additional snow or rain on top of the remaining snowpack. These systems may cause substantial runoff and the beginning of minor flooding in the southern headwater portion of the Red River of the North, eventually leading to major flooding sometime from the last week of March through early April.

Causes for Spring Flooding

Spring flooding is caused by a variety of factors, including heavy late summer and fall precipitation, which leaves soils saturated and streams running high before the winter freeze; heavy winter snowfall resulting in deep snowpack; stable below-freezing temperatures throughout the winter delaying snow melt; frozen and/or saturated ground, which inhibits infiltration of water into the soil; rapid snowpack melt due to warming springtime temperatures; backwater flooding due to ice jams; and heavy spring rainfall accelerating snow melt and adding to the high volume of water already in river systems. The rate at which temperatures warm this spring, along with the impacts of any additional precipitation, will determine the magnitude, timing and extent of the flooding.

National Weather Service models show this year’s snowpack in the north-central U.S. contains a water content ranked among the highest of the last 60 years.

Heavy rainfall at any time can lead to river flooding, even in areas where overall river flood potential is considered below average. Find current hydrologic information specific to your area online.

Floods are the deadliest weather phenomena – claiming an average of 100 lives annually. Many of these deaths occur in automobiles and are preventable. If confronted with a water-covered road on foot or in an automobile, follow National Weather Service advice: Turn Around, Don’t Drown.

Other aspects of the U.S. spring outlook for April through June include:

Temperature:Odds favor above-average temperatures in much of the southern half of the U.S., and below-average temperatures from the Pacific Northwest to the northern plains.

Precipitation:Odds favor drier-than-average conditions from south Florida and along the Gulf Coast through Texas and into the Southwest.Wetter-than-average conditions are favored across parts of the northern plains.

Drought:From the Southwest, across the South and northward to the mid-Atlantic, drought has been spreading and deepening since the winter and is forecast to persist in spring. Wildfires will be an increasing threat, especially when humidity is low and when winds are high.